Indirect cooling system for an electrical device

ABSTRACT

A cooling system for indirect cooling of a device, in particular a superconducting device, located in a vacuum chamber includes at least one refrigerating machine component. This refrigerating machine component includes an ambient temperature section and a low-temperature section, projects into the vacuum housing, is secured thereto via spring elements and, at its low-temperature end, is heat conductively connected to the device to be cooled. To reduce the vibrations transmitted to the device, the ambient temperature section of the refrigerating machine component may be arranged in the evacuatable compartment of a housing unit rigidly connected to the vacuum chamber.

FIELD OF THE INVENTION

The present invention relates to a system for indirectly cooling anelectrical device. In particular, the present invention relates to asystem for indirectly cooling a superconducting device, to be kept at alow temperature, which is located in an evacuatable internal compartmentof a vacuum chamber.

BACKGROUND INFORMATION

Electrical devices, in particular, superconducting devices to be cooledto low temperatures, such as the winding of a magnetic coil or agenerator, or a superconducting cable, require cooling systems ensuringthe operability of the components to be cooled at the low operatingtemperature. Bath cooling, forced cooling or, in particular, indirectcooling can be used to cool these components.

Indirect cooling allows relatively compact, coolant-free cryostats to bebuilt without coolant containers and frees the user from having toreplenish the cryofluid. The required cooling effect can be achievedusing a cryocooler, normally designed as a dual-stage cooler, whichoften works by the Gifford-McMahon principle. With such a cryocooler,the first stage may have a typical cooling capacity of 50 W atapproximately 60K in the first stage and 1 W at 10K in the second stage.

Indirect cooling can be advantageously provided for superconductingmagnet systems used for nuclear spin tomography. The correspondingcooling system must be designed so that as little as vibration aspossible is transmitted to the magnet system when the refrigeratingmachine or a refrigerating machine component is thermally coupled to thesuperconducting magnet system. All conventional refrigerating machineshave mechanically movable components causing considerable vibrations inthe frequency range of 1 to a few tens of Hz. The pressure fluctuationsof the working medium, typically helium at approximately 20 bar, canalso contribute to the vibrations. If these vibrations act on the magnetsystem without being damped, undesirable eddy currents appear as themagnet system generating a basic magnetic field with an induction of 1T, for example, is operated. These eddy currents not only increase theheat load on the refrigerating system, but also interfere with theimaging system of the nuclear spin tomography machine.

In order to solve the problems concerning transmission of vibrations, ina refrigerating system described in European Patent Application No. 0260 036 for the He-cooled superconducting magnet of a nuclear spintomography system, the magnet and a surrounding radiation shield arecoupled to components of a refrigerating machine via flexible connectingelements made of a heat-conducting material. The damping characteristicrequirements of such a coupling, also acting mechanically between amagnet and a refrigerating machine, are however, in general,considerably higher in the case of magnets for nuclear spin tomography.

U.S. Pat. No. 5,129,232 also describes a cooling system for thesuperconducting magnet of a nuclear spin tomography system withappropriate vibration-damping heat-conducting connecting elementsbetween a refrigerating machine and a radiation shield/superconductingmaterial. To further improve the vibration damping, the refrigeratingmachine is supported by the vacuum chamber that surrounds thesuperconducting winding via spring elements. These spring elements notonly have to bear the weight of the refrigerating machine itself, butalso the force of the external atmospheric pressure acting upon theambient temperature section of the refrigerating machine. This pressureforce is caused because the ambient temperature section is under thenormal pressure surrounding the vacuum chamber of the superconductingwinding, while the low-temperature section of the refrigerating machineis in an evacuated housing unit, which projects into the vacuum chamberof the superconducting magnet. Therefore, the spring elements arepressed together with a relatively great force and therefore must have amatching elastic force. The rigidity of the springs is, therefore, alsohigh, so that vibration damping by the conventional spring elements islimited accordingly.

SUMMARY OF THE INVENTION

In accordance with the present invention, a system for indirect coolingof an electrical device is provided. In particular, a superconductingdevice, to be kept at low temperature, is located in an evacuatableinternal compartment of a vacuum chamber. The system contains at leastone refrigerating machine component, which has an ambient temperaturemachine section and a low temperature machine section located in anevacuatable compartment. The refrigerating machine component movablyprojects into the vacuum changer through an opening in the vacuumchamber. The refrigerating component is also elastically secured to thevacuum chamber through a spring element so that it seals the vacuumchamber opening. Additionally, the refrigerating machine component isheat conductively connected at its low-temperature end to the electricaldevice.

An object of the present invention is to improve the cooling system sothat the transmission of vibrations from the corresponding refrigeratingmachine or refrigerating machine components to the electrical device tobe cooled is further reduced.

This object is achieved according to the present invention by thepositioning that the ambient temperature section of the refrigeratingmachine component in an evacuatable compartment of a housing unitrigidly secured to the vacuum chamber.

One advantage of this design of the cooling system is that, byevacuating the ambient temperature section of the compartmentsurrounding the refrigerating machine, the force of the externalpressure no longer acts upon the spring elements. The spring constant ofthe spring system made up of the spring elements can thus be reduced toa fraction of the value that would be required for vibration dampingwithout evacuation. This results in a corresponding increase invibration damping.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cooling system according to the present invention.

FIG. 2 shows the cooling system with an open heat switch.

FIG. 3 shows the cooling system with a closed heat switch.

DETAILED DESCRIPTION OF THE INVENTION

Due to the vibration-damping support or suspension of the refrigeratingmachine or a machine component, the cooling system according to thepresent invention can be provided to particular advantage for electricaldevices to be cooled to low temperatures that are sensitive tovibrations caused by refrigerating machine components. Such devicesinclude, for example, the superconducting magnet system of a nuclearspin tomography machine. The cooling system can also be used with otherelectrical devices to be cooled to low temperatures.

FIG. 1 shows a cross section of a component of a cooling system 2designed according to present invention. The components not shown inFIG. 1 and not explained in detail in the following description aregenerally known in the art. The system allows a coolant-free cryostat tobe designed.

The cooling system 2 shown in FIG. 1 includes at least one refrigeratingmachine 3 with at least one refrigerating machine component 4, which mayhave two cooling stages 5 and 6. Refrigerating machine 3 can be, forexample, a Gifford-McMahon type cryocooler. Other single- or multistagerefrigerating machine types can also be used. Refrigerating machinecomponent 4 or the entire refrigerating machine comprises an ambienttemperature section 4a located in the ambient temperature area RT, and alow-temperature section 4b, extending to the low-temperature area TT,and comprising cooling stages 5 and 6. Low-temperature section 4bprojects into an evacuatable compartment 9 of vacuum chamber 8 throughan opening 7 in the housing; vacuum chamber 8 is evacuated to a residualpressure p1 of an insulating vacuum. Opening 7 is dimensioned so thatthe low-temperature machine section 4b can move somewhat displaceably inits vertical direction. At the low-temperature end of second coolingstage 6, section 4b is heat-conductively coupled to a device 10 to becooled, for example, a superconducting magnet. FIG. 1 only shows anupper portion of a structure to be cooled of this magnet, for example,its housing, surrounded by an insulating vacuum.

The low-temperature section 4b of refrigerating machine component 4 ispreferably located in its own housing unit 12, whose internalcompartment 13 can be evacuated. In order not to vent the entire vacuumsystem of vacuum chamber 8 to normal pressure p0 of the surrounding ofcooling system 2, thus making it necessary to warm up and cool downmagnet 10 to be cooled to a low temperature in a time-consumingprocedure taking one week, for example, low-temperature section 4b ofrefrigerating machine component 4 may be installed in a separatevacuum-tight lock insulated against interior compartment 9 of vacuumchamber 8. This lock, which may contain thin-walled VA tubes and whosevolume is not much greater than that of refrigerating machine section 4bneeds to be, allows access to internal compartment 13 from the outsideor the top. In the area of the position of the first and second coolingstages 5 and 6, heat-conducting connecting pieces 15 and 16 are providedon the outside of housing unit 12 and mechanically detachable heatcontacts 17 and 18 are provided on the inside. These heat contacts canbe formed with elastic contact plates made of Cu, which may be gold- orsilver-plated and/or indium-plated. They allow heat transfer from therespective cooling stage of low-temperature section 4b of refrigeratingmachine component 4 to the thermal connecting pieces 15, 16 through thewall of housing unit 12. In the exemplary illustrated in FIG. 1, such aswitchable heat contact is implemented in the radial direction. Such aheat contact is to be provided from the first and second cooling stages5, 6 to a radiation shield 20 and the structure of magnets 10 via heatcontacts 17, 18, thermal connecting pieces 15, 16, and flexible thermalconnecting elements 21, 22. The flexible thermal connecting elements canbe copper cords or strips, through which hardly any vibration ofrefrigerating machine component 4 is transmitted. In operation, housingunit 12, working as a lock, is evacuated to a residual pressure p2. Itcan be vented or evacuated for replacing refrigerating machine component4 at inlet 24 of the low-temperature housing unit 12.

Refrigerating machine embodiments where the low-temperature section 4bis not arranged in its own evacuatable compartment 13 of a specialhousing unit, but projects directly into the inner space 9 of vacuumchamber 8, are also conceivable. In any case, opening 7 and theevacuatable space 13 that may be present are sealed in a vacuum-tightmanner by the support or suspension of refrigerating machine component4.

In order not to expose the ambient temperature section 4a ofrefrigerating machine component 4 and, thus, the support or suspensionof this component to the external atmospheric pressure, ambienttemperature section 4a is arranged in a separate evacuatable housingunit 26 according to the present invention. This housing unit enclosesthe ambient temperature section 4a of refrigerating machine component 4and is rigidly and hermetically secured to the outside of vacuum chamber8. Its compartment 27 can therefore be evacuated to a residual pressurep3 or vented separately from the insulating vacuum of magnet 10 and ofthe low-temperature machine section 4b through an inlet 28. When vented,refrigerating machine component 4 presses against vacuum chamber 8 notonly with its own weight Gk of approximately 200 N, for example, butalso with force Lk of the external atmospheric pressure. This means thatfor a diameter of approximately 160 mm of refrigerating machinecomponent 4, an additional force Lk of approximately 2 kN, i.e., about10 times the force of gravity Gk, appears. This force Lk is absorbed inconventional cooling systems (see U.S. Pat. No. 5,129,232) by a suitablyrigid spring system, which should dampen the transmission of vibrationsof the refrigerating machine component to the device 10 to be cooled. Inthe cooling system 2 according to the present invention, it isadvantageous if the spring system is designed so that practically onlythe force of gravity Gk of refrigerating machine component 4 isabsorbed. For this purpose, the spring support illustrated of therefrigerating machine component comprises spring elements 30, parallelto which elastic dampening elements 31 may be arranged. Elements 30 and31 are mounted between vacuum chamber 8 and support extensions 32 thatextend parallel thereto and are rigidly secured to refrigerating machinecomponent 4, in particular to the area of connection between ambienttemperature section 4a and low-temperature section 4b. Supportextensions 32 and elements 30, 31 not only serve for support orsuspension, as the case may be, but also for sealing interior space 9 ofvacuum chamber 8 in the area of opening 7.

When housing unit 26 is vented, the support of refrigerating machinecomponent 4 elastically vibrates, due to the effect of vacuum on itslow-temperature section, up to a fixed mechanical stop 33. The vibrationis only damped when the compartment 27 of housing unit 26 is evacuatedto an operating pressure p3, of less than 100 mbar, for example. Thetypical pressure is approximately 10 mbar. Force Lk of the atmosphericpressure is reduced to approximately 20 N due to the evacuation. In thisstate, refrigerating machine component 4 is elastically supported byelements 30, 31. The corresponding spring constant can therefore bereduced by a factor of 1/10 compared to the value that would be requiredfor vibration damping without evacuation. The resulting soft suspensionallows, in many applications, refrigerating machine component 4 to bemounted directly on a housing component of a device to be cooled to alow temperature, such as a magnet, without additional mechanical andheat-conducting elements being required. The FIG. 1 indicates flexibleconnecting pipes 35 for the ambient temperature section 4a of therefrigerating machine, extending in a vacuum-tight manner throughcompartment 27 of the ambient temperature housing unit 26, for example,for helium and electrical connecting cables.

In a conventional Gifford-McMahon refrigerating machine 3, therefrigerating capacity of second stage 6 of refrigerating machinecomponent 4, to which the device to be cooled, for example, magnet 10,is thermally coupled, is approximately 1/5 of the refrigerating capacityof first stage 5. The heat capacity of a superconducting magnet,contributes at least 2/3 to the thermal mass to be cooled in a typicaldesign. To cool a superconducting magnet from ambient temperature tooperating temperature with the help of a refrigerating machine alone, itis therefore advantageous to use the relatively high refrigeratingcapacity of the first stage 5 of the refrigerating machine to precoolthe magnet. This requires a detachable thermal contact, which firstestablishes a thermally conductive connection between the first coolingstage and the magnet for cooling and is only opened at a temperaturelevel close to the final temperature of the first stage. The magnet thenreaches its operating temperature with the refrigerating capacity of thesecond stage. Have a very low heat conductivity is required when thethermal contact is open, since a heat flow leak through this contactwould represent a load on the second stage. An exemplary embodiment of asimilar detachable heat contact is shown in FIGS. 2 and 3, FIG. 2showing the contact closed and FIG. 3 shows the contact open. The heatcontact shown in FIGS. 2 and 3 and denoted as 40 is formed by athermally conductive contact plate 41, located between a supportingstructure 43 rigidly connected to device 10 to be cooled and a componentof the low-temperature section 4b of the refrigerating machine, kept atleast largely at the temperature of the first cooling stage. Thiscomponent of refrigerating machine section 4b can be formed by thermalconnecting piece 15, for example. Since this connecting piece is rigidlyconnected to refrigerating machine section 4b or housing unit 12 thatsurrounds it, it follows the excursion of spring elements 30, 31. Duringcooling from ambient temperature, compartment 27 of external housingunit 26 is first vented. Due to pressure conditions p0, refrigeratingmachine component 4 is pressed by the external atmospheric pressureagainst the soft support via spring elements 30, 31 in the direction ofmagnet 10 with force Lk, until thermal contact 40 of the first coolingstage 5 reaches its mechanical stop (see FIG. 2). This stop is formed bycontact plates 41 on support structure 43, rigidly connected to magnet10. Due to the evacuation of compartment 27 to pressure p3 after themagnet has been precooled approximately to the temperature of firstcooling stage 5, force Lk no longer acts on refrigerating machinecomponent 4, so that spring elements 30, 31 elongate with the remainingforce of gravity Gk. Connecting piece 15, rigidly connected torefrigerating machine component 4, is lifted from plates 41 to a degreecorresponding to this displacement, so that thermal contact 40 isopened.

When thermal contact is open (see FIG. 3), full isolation of device 10to be cooled and the support structure 43 connected to it from the firstcooling stage 5 of refrigerating machine section 4b, is guaranteed sothat no thermal load is placed on second cooling stage 6 of this sectionby heat leaks from the warmer first stage. Compared with theconventional gas heat switches, this arrangement can be relativelycompact, yet allows good thermal conductivity.

FIGS. 1 through 3 show a support according to the present invention of arefrigerating machine or a component thereof. A suspension using springelements that are not to be affected by force Lk of the atmosphericpressure acting upon the machine section at ambient temperature is alsoconceivable.

What is claimed is:
 1. A system for indirectly cooling an electricaldevice in a vacuum chamber, the vacuum chamber including a firstevacuatable compartment, the electrical device positioned in the firstevacuatable compartment, the vacuum chamber further having an opening,comprising:at least one refrigerating machine component including anambient temperature machine section and a low-temperature machinesection, and having a low-temperature end, the low-temperature machinesection positioned in the first evacuatable compartment, the at leastone refrigerating machine component movably projecting into the vacuumchamber through the opening, the at least one refrigerating machinecomponent being elastically secured to the vacuum chamber via at leastone spring element and hermetically sealing the opening of the vacuumchamber, the low-temperature end of the at least one refrigeratingmachine component being heat conductively coupled to the electricaldevice; and a housing unit coupled to the vacuum chamber, the housingunit having a second evacuatable compartment, the ambient temperaturemachine section being arranged in the second evacuatable compartment. 2.The system according to claim 1, wherein the electrical device is asuperconducting device.
 3. The system according to claim 1, wherein thefirst evacuatable compartment surrounds the low-temperature machinesection.
 4. The system according to claim 1, wherein the secondevacuatable compartment surrounds the low-temperature machine sectionand projects into the first evacuatable compartment, the secondevacuatable compartment being vacuum-tight isolated from the firstevacuatable compartment, and wherein the housing unit is a lockaccessible from the second evacuatable compartment.
 5. The systemaccording to claim 4, further comprising:at least one cooling stagethermally connected to at least one component of the housing unit usingat least one detachable heat contact.
 6. The system according to claim1, wherein the at least one spring element supports the at least onerefrigerating machine component.
 7. The system according to claim 1,wherein the at least one spring element suspends the at least onerefrigerating machine component.
 8. The system according to claim 1,further comprising:at least one elastic damping element positionedparallel to the at least one spring element.
 9. The system according toclaim 8, wherein at least one of i) the at least one spring element, andii) the at least one elastic damping element, seal the opening of thevacuum chamber to the at least one refrigerating machine component. 10.The system according to claim 1, wherein the low-temperature machinesection includes at least two cooling stages.
 11. The system accordingto claim 1, further comprising:a stop element limiting the excursion ofthe at least one spring element when the second evacuatable compartmentis vented.
 12. The system according to claim 11, wherein the stopelement is a thermal contact establishing a heat conducting connectionbetween a first cooling stage and the electrical device only when thesecond evacuatable compartment is vented.
 13. The system according toclaim 1, further comprising:at least one elastic connecting elementthermally coupling the low-temperature end of the at least onerefrigerating machine component to the electrical device.